Power supply mains usually provide an AC voltage, for example 230 volts. In central Europe, the frequency of the AC voltage (mains frequency) is 50 Hz. Both the amplitude of the voltage and the mains frequency may be subject to fluctuations. Mains analyzers detect the amplitude of the AC voltage. This requires the mains analyzers to know the mains frequency as exactly as possible.
To derive the mains frequency in a mains analyzer, the voltage zero crossings are usually evaluated in a received voltage signal. If implausible values are eliminated, it is possible to derive the period of the AC voltage from the zero crossings, with mains analyzers checking a relatively large number of periods and forming an average.
If the voltage signal has a high level of harmonics, there are additional zero crossings. These make it difficult to derive the period duration. For this reason, mains analyzers often dispense with accurate frequency detection when they receive signals loaded with harmonics. Instead, a nominal frequency is taken as a basis for the further data capture. This harms the accuracy of the actual measurements by the mains analyzer.